I've had a number of mandarins over many years, young and old, single and paired, I've never seen them float at the surface, so I would not see that as normal.
I do see my mated pairs do their spawning dance as the lights go dim, and that brings them to the surface briefly, but other than that I've just never seen them just hover at the surface, wonder why yours is doing this?
I see another post basically saying same as you, is that your post also?